The present invention relates to a device for isolating and feeding the lowest sheet in each case from a stack of flat substantially planar items such as paper sheets or the like.
A device of the type is known, for example, from EP 0 464 578 A1 and serves for isolating boards which are fed to a subsequent unit for further processing. Possible further-processing operations are printing, cutting-to-size or division into a number of individual sheets, fluting or grooving and milling, the manufacture of book covers, file covers, etc. This device has a transporter which can be moved forwards and backwards and which is driven by a crank-slide mechanism with coupler-mechanism expansion with a sinusoidal movement pattern. Fastened to the transporter are pushing-out elements which are dimensioned in such a way, in terms of their height, that they grip only the lowest sheet at the rear edge during the forward movement and push it through an admission aperture formed by a base plate and a front-edge stop. In the process, the sheet is pushed into a pair of drawing-off rollers which takes over the transportation of the sheet from then on and feeds it to the subsequent processing unit, while the transporter with the pushing-out elements returns to its starting position for the purpose of pushing out the next sheet. From DE 75 15 810 U1, a device is known in which a suction device which acts upon the front region of the lowest sheet is additionally provided for the purpose of isolating and feeding curved materials.
The sheets rest against a stationary edge with their front edge. Accordingly, as the sheet length becomes greater, the rear edge is displaced rearwards relative to the front edge stop and to the pair of drawing-off rollers. For the purpose of setting different sheet lengths, the transporter with its appertaining drive is received in a carriage which can be brought, via adjusting spindles, into the pushing-out position which corresponds to the sheet length. For the purpose of driving the crank-slide mechanism, a rotating movement is tapped off, via a sliding sleeve, from a spline shaft aligned along the direction of displacement, and is reoriented via a bevel-gear mechanism. The prior art mentioned also indicates an alternative form of embodiment for adjusting the pushing-out position. In that embodiment, the mechanism for generating the forward and backward movement is disposed in a stationary manner. It actuates a sliding carriage which has a transporter which can be adjusted via adjusting spindles. The rotational movement for adjustment purposes is transmitted in known manner via a sliding sleeve from a profiled shaft to the moving system of the sliding carriage.
Because of the disposition of the adjusting system on the moving system, the latter is burdened by additional masses for which allowance has to be made in the guides of the carriage and also in the drive and which, moreover, restrict the maximum isolating output. The driving connection of the adjusting system via sliding sleeves is exposed to constant wear. Although the moving mass is reduced to a minimum in the aforesaid form of embodiment, use is nevertheless made of a very complicated driving system which has, in the sliding sleeve, a driving member which is subject to play and wear.
The sinusoidal driving movement of the transporter which is generated via the crank-slide mechanism or, in other embodiments, via a crank mechanism has ranges of maximum, near-constant speed during both the forward and the backward movements. The taking-over of the sheet during the forward movement through the pair of drawing-off rollers ideally takes place within this speed range, and that at a speed which is synchronous with the said pair of rollers. In order to achieve high isolating outputs and/or in the case of large sheet lengths, however, the drawing-off speed is set so as to be substantially higher. This leads to the sheets being pulled in a skewed manner, which jeopardizes further processing of a good quality. Because of the fixed period of the sinusoidal driving movement, the sheets are fed to the pair of drawing-off rollers at a fixed clock-pulse interval. This is also necessary for some further-processing apparatuses. During cutting-to-size or division into a number of blanks, on the other hand, pushing-out of the sheets which is adapted to the sheet length is appropriate in order to achieve maximum isolating outputs, something which can be achieved only by means of major structural expense in the designs of drive indicated.